1. Technical Field
This invention relates to means for reducing stress and torque associated with pumping operations for the production of oil from a non-flowing oil well. In particular, the invention relates to a stress and torque reducing tool (hereinafter frequently referred to as "STR TOOL") employed to reduce damage to subsurface equipment.
In the production of petroleum fluids from oil wells which do not flow, it is a common practice to employ a reciprocating pump which is located deep within the well bore and which is operated by means of a string of sucker rods extending to the ground surface. The upper or polished rod section of the sucker rod string is ordinarily reciprocated by a pump operating unit consisting of a walking beam, one end of which is connected by a horsehead sling to the polished rod and the other end is connected by a Pittman to a rotating crank which is driven by a suitable prime mover, such as an electric motor.
In the operation of reciprocating pumps through the use of sucker rods for the production of oil and associated gas, the sudden reversal movement at the end of the upstroke and the downstroke of the sucker rods imposes reversal strains on the sucker rods as the stretch of the sucker rods is either suddenly released or suddenly imposed upon the rods.
In addition, the subsurface pumps used in connection with the sucker rods can undergo what is referred to as gas lock. This is a condition which occurs when gas enters the area below the plunger of a conventional pump, when the plunger is at the uppermost position of its normal travel and while traveling through its lowermost point of travel, cannot compress the gas to a pressure sufficient to force the traveling valve open. On the following upstroke, the gas expands keeping the pressure high enough in the area below the plunger that the standing valve will not open and allow fluid to enter the pump. This compressing and expanding of gas repeats itself on each downstroke and each upstroke without increasing pressure enough to open the traveling valve, or decreasing in pressure enough to allow the standing valve to open and allow fluid to enter the pump.
Another condition referred to as "fluid pound" occurs when the plunger of the conventional pump is at its uppermost point of travel, and not enough fluid enters the pump to completely fill the area vacated by the plunger, and the plunger while moving on its downstroke impacts the fluid which is substantially incompressable, thus severely jarring the sucker rods and pump. Such condition could occur on every downstroke of the pump and sucker rods. A similar occurrence called a "gas pound" occurs when the plunger on the downstroke compresses gas to a pressure greater than the rod weight but not sufficient pressure to open the traveling valve.
2. Background Information
Conventional pumping devices in oil wells frequently develop gas locks. The solution to this problem in the past has been to space the pumps where it actually bumps bottom on every stroke to eliminate gas locks. This action of bumping bottom causes many destructive effects. It increases the stress range on sucker rods. It causes the rods to go into the compression state each and every time the pump bumps the bottom. It also causes the rods to buckle and slap the inside of the tubing, which causes increased wear to the rods, rod couplings and tubing. When the pump bumps bottom and causes the entire weight of the rods to be transferred to the tubing string in a shock load, which can cause premature failure of tubing couplings and threads, such shock loads are also transferred to the pumping unit when the pump bumps bottom resulting in premature failure of structural bearings and torque reversals in the gears of the gear box causing excessive wear on the gear teeth and gear box bearing. Unintentional gas or fluid pounding in operation of conventional pumps is a common problem in low fluid level wells and marginal producing wells. In order to reduce strain on the sucker rods and the pump, one approach outlined by Hilton (U.S. Pat. No. 2,674,956) is to provide one or more stagers or valve assemblies mounted intermediate the sucker rod string which close on the upstroke and open on the downstroke so that the static head on the traveling valve in the pump will be reduced by transferring a portion of such head to the stagers. In addition, Hilton provides an arrangement to retard the downward movement of the sucker rods by forming buffers which restrict the flow of liquid through the corresponding stager valves and afford somewhat of a buoyant effect for the sucker rods. Other techniques for handling gas locks have been developed such as those proposed by Spears Specialty Oil Tool of Tomball, Tex. who according to their literature have developed a tool referred to as a "Buster" that uses hydrostatic head in the tubing to load or cock itself on the upstroke and as the sucker rod reverses to start the downstroke, the Buster releases and bumps the top of the plunger. This action unseats the traveling balls allowing compressed gas to be transferred out of the pump into the tubing in a controlled fashion. The same company has devised a valving system which unseats the traveling valve. This company refers to the tool as a "Sidekicker" and advertised as a pending application. Spears (U.S. Pat. No. 4,332,533) describes a fluid pump which is intended to be more efficient and thus overcome some of the problems associated with gas pound, gas lock and so forth. Chancellor (U.S. Pat. No. 3,292,552) mentions the problem of gas locking in pumps used in producing high viscosity oil from wells.